Final update, and thank you for all your help!

[Jackaroo]

Nannynick - I spoke to lots of people this morning, and finally the other mum again... she has been using the expression "childminder", but actually she just meant that the au pair will be doing the childcare approval scheme course, and paed.s first aid course... which of course means she can be a registered nanny! Other mum seems to think she can be self-employed with this qualification/registration, which I'm yet to get to the bottom of, but frankly I suspect we'll do it as her employers as it's only for another year, and I'd rather know it's all sorted!!
Thank you so much for listening to all my babble, I was getting sooo worried (like when I have to fill in a tax return lol!), and it really calmed me down, and made me braver about talking to her....

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Good to hear you have sorted it out, or at least now have it sorted out in your mind about what was going on and the implications of that change. Stick with being employer, take advantage of being able to pay via childcare vouchers (if you employer provides them). With luck it all run smoothly for the next year.